1. Field of the Invention
The invention relates to integrated optical waveguides with lateral confinement, the method of their manufacture and their use in integrated optics.
Optic waveguides are coming into increasing use not only for data transmission but also for data processing as they enable high-speed processing. This data processing aspect has given rise to what is called "integrated optics" technology relating to light propagation in dielectric waveguides made in planar shape on transparent substrates.
2. Description of the Prior Art
The standard way of making lateral confinement optical waveguides is to make them on a monocrystal lithium niobate surface by doping the crystal, generally with titanium introduced by thermal diffusion. The doped region has its refraction index increased and therefore becomes a guide for light in a certain range of wavelengths.
When guiding, with confinement in two dimensions, is desired, a titanium strip is diffused. This titanium strip has an identical pattern to the desired path of light. A doped region with an index greater than that of the substrate is obtained. This doped region is similar to an optical fiber core, with a guiding path of the desired geometry on the crystal surface.
Additional methods may be used, either to lower the refraction index on either side of the guide and prevent the formation of a guide with a uniform surface or to raise the index within the guiding zone, also according to a particular profile.
However, when titanium is diffused thermally in lithium niobate crystal, several effects may contribute to developing electrical charges on its surface:
the pyroelectrical effect which causes a flow of electrical charge to appear along an optical axis when the crystal is affected by a temperature variation; PA1 the piezoelectrical effect or appearance of a voltage when mechanical deformation affects the crystal, for example during heat expansion.
These charges may be drained by the titanium strips to be diffused. But, in general, these strips have gaps ranging from a few microns to about ten microns. These gaps are due either to lithographic imperfections or to the pattern itself. In this case, instead of enabling the charges to be neutralized and therefore, the potential to become uniform, the strips may cause major differences in potential on either sides of their gaps. The very high local electrical field may then cause localized destruction of the crystal if dielectrical rigidity is inadequate. A splitting of the crystal is then observed. This causes a break in the guide and leads to additional propagation losses.
Furthermore, at the same time that the guide is made by diffusion of the titanium strip, an unwanted plane guide is created by the exodiffusion of lithium oxide during the thermal diffusion cycle.
Finally, an essential aspect of the value of waveguides of this type, made of lithium niobate, is that they exhibit Pockel's effect. This means that the propagation constant of the guided wave, i.e. the phase of the wave, can be modulated by applying a time-variable electrical field to the guide. Special configurations of guided circuits can be used for the amplitude modulation of light in amplitude to switch over light signals, perform signal processing functions, etc. The electrical field is applied by means of planar electrodes placed in such a way that the optical guide undergoes the desired variation in refraction index. For this, the position of the electrodes and the crystallographic orientation of the substrate are adapted to the desired functions.
However, it has been experimentally observed that the electrical field created between the electrodes inside the guide is not homogeneous, since the material crossed is anistropic and not homogeneous because of the thermal diffusion process used. With planar electrodes placed at the edges of the guide, in certain configurations, the field lines obtained go through the doped region and the non-doped region, and a relaxation phenomenon then comes into play if the material has regions with dielectrical constants and different resistivities. Furthermore, the distribution of electrical potentials between these two regions varies according to the frequency used. Consequently, the electrical field effectively applied to the guide varies with the frequency. This implies a variation in the efficiency of the apparent electro-optical modulation which comes into play at about 1 KHz and which can reach a factor of 1.5. This phenomenon is particularly harmful to low-frequency uses, for example, for the encoding of signals in underwater monitoring, or uses requiring a large band for signal processing in fiber gyrometry for example.